Either the J12YC or the J14YC. Either will work fine, whichever is easier to get. Skip the resistor plugs. What I found with those webers is that the ported vacuum for the vacuum advance does not provide enough signal. Keep in mind that the original carb used ported vacuum for the advance, that is the port was to atmosphere (above the throttle blade). When the throttle blade began to open, vacuum built as the air passed the port, allowing the advance to start opening, reached max signal and max vacuum advance, and then tapered off as the throttle blade opened fully, eliminating the vacuum advance. So with a stock carb, when you gave the gas a rap at idle, the distributor would move quite a bit. Because the port is ported vacuum, you can set the timing at idle without disconnecting the advance line because there is no vacuum (or very little) to the advance unit at idle.
With the webers, the insufficient signal from the weber's vacuum port impedes proper advance. So generally you have to run the advance line to full manifold vacuum (where the wiper motor hose is). Thus, you set the timing with the vacuum hose disconnected and after reconnect the hose.
As for jetting, couple of places sell them, Redline, Sterling, same jets as the 32/36 and 38/38 weber. Keep in mind that for the Weber, there is an idle jet and an idle air jet. Both play a part in idle mixture. Also emulsion tubs but you dont even want to go messing with them unless absolutely necessary. The mixture screw only adjusts for volume. So if the idle jet is too large, and the idle air jet is too small, it will always run rich no matter how much you turn the mixture screw in. Worse, the main jetting plays a part on the off idle circuit with these webers, which in turn affects the quality of the idle fuel. They are very adjustable, but you need a good bit of patience to tinker with them. I am still learning with my dual 38/38s and I won't lie, the curve is steep and sometimes frustrating.
If you really want to get this truck running well, at least in my opinion, I would get back to basics. Put the single bbl carb back on (confirming that the power piston is not sticking), or get a Carter replacement carb (I always had very good luck with the YF's) and start from that point as those carbs were designed and engineered for these motors. Then fresh plugs, and just for giggles and because they are cheap, replace the condenser. As a couple of the fellas here have noted, today's stuff can be junk right out of the box.
Then confirm your timing, adjust the carb, and start from that basic point. Like I said, I myself am not a fan of a single weber on a 235 single bbl manifold. I am not saying that is your problem, but too many rabbits going down too many holes, in my view anyhow.
